The present invention relates generally to ink jet printing, and more specifically to a method for operation of an ink jet printing head of the type wherein printing ink is discharged toward a writing surface by the aid of an electric field established between electrodes.
Various types of ink jet printing heads are known. One example of ink jet printing heads is disclosed in U.S. Pat. No. 4,555,717, illustrated in FIG. 1 of the present application, the multi-nozzle ink jet printing head comprising an air-ink nozzle plate 8 having a plurality of air-ink nozzles 1 successively arranged in a row and an ink nozzle plate 9 having a plurality of ink nozzles 2 successively arranged in a row and aligned with the air-ink nozzles 1 with one-to-one correspondence therebetween. A common electrode 3 is attached to a surface of the air-ink nozzle plate 8 and a plurality of control electrodes 4 are provided on a surface of the ink nozzle plate 9 in association with the plurality of ink nozzles 2. The common electrode 3 and control electrode 4 are provided for establishing an electric field therebetween to cause ink droplets held in the ink nozzles 2 to extend toward the air-ink nozzles 1 and to be carried by airstream supplied from a source of pressurized air and discharged through the air-ink nozzles 1.
One current method for the operation of such a multi-nozzle ink jet printing head involves a technique in which a pulse signal indicated by (a) of FIG. 2 is independently applied to each of the plurality of control electrodes 4 and a signal indicated by (b) is applied to the common electrode 3. The pulse signal (a) comprises pulses with a predetermined voltage Vs and different width (each will be hereinafter referred to as control pulse). The signal (b) is produced through superimposition of a pulse train with amplitude Vpb (which will be hereinafter referred to as bias pulse train) on a DC voltage Vb. In response to application of the control pulse thereto, printing ink is discharged toward writing paper wrapped around a cylindrical drum, for example.
The frequency of the bias pulse train superimposed on the DC voltage Vb is set to be equal to that of an input signal inputted to the ink jet printing head, i.e., a picture element signal, and the voltage Vpb or pulse width of the bias pulse is determined so that printing ink is not discharged in the case of absence of the control pulse. Generally, as the voltage Vpb or width of the bias pulse is closer to a limit value at which the discharge of ink is started, the discharge thereof is made easier, thereby enabling reduction of the voltage Vs of the control pulse and improving the response characteristic of the ink discharge. Therefore, it is desirable that the voltage Vpb or width of the bias pulse is set as close to the limit value as possible. However, the ink jet printing head is susceptable to the influence from mechanical impact and elecrical noise. When the ink jet printing head is adapted for a serial printer or the like, it will be subject to a mechanical impact as it is turned back after reaching an end of recording paper. If the occurrence of the mechanical impact is concurrent with the application of the bias pulse Vpb to the ink ject printing head, ink droplets held in the ink nozzles 2 are apt to be discharged in response to the mechanical impact irrespective of absence of the control pulse to be fed to the control electrode 4. Furthermore, the discharge of ink droplets will occur in response to the generation of a very low level noise when the pulse voltage or pulse width is set to a value extremely close to the limit.
On the other hand, a minimum control pulse voltage Vs which makes possible the discharge of ink from the ink nozzle 2 (which will be hereinafter referred to as threshold level Voh) depends on the width of the control pulse required for recording one dot on a writing surface (which will be hereinafter referred to as minimum control pulse width Pwmin). Namely, the shoter the minimum control pulse width Pwmin, the higher the threshold level Voh. For example, when Pwmin=300 .mu.s, Voh=300 V, and when Pwmin=200 .mu.s, Voh=380 V. The frequency fp of picture element signal is determined in accordance with the minimum control pulse width Pwmin, that is, Pwmin=1/fp. For example, when Vs=300 V, Pwmin=300 .mu.s, and therefore the maximum frequency of the picture signal is 3.3 kHz. In order to further increase the frequency of the picture element signal from the viewpoint of increase in recording speed, it is required to make greater the control pulse voltage Vs. However, the increase in the control pulse, voltage Vs results in a high manufacturing cost and reduction in accuracy of operation.